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Original question:

TS5A63157: Tj(max)

TS5A63157: for I2C protection

BO
Genius 12760 points
Part Number: TS5A63157
Other Parts Discussed in Thread: TMUX1511, PCA9306

We are trying to use this IC as a protection of I2C bus, in case of DSP power supply failure. We are using two pcs, for SDA and SCL line and the intention is to use “Isolation in Powered-Off Mode” feature.

 

  1. If we apply 3.3V on S_SDA while S_B3V3 = 0V (turned off unit connected to comm bus), everything works ok, bus is protected, I2C voltage levels are not affected
  2. The unit is operating S_B3V3 = 3.3V and goes to turn off state S_B3V3 = not supplied anymore. During this transition IC stays in a conducting mode -> thanks to this revers conduction is S_B3V3 = 1.5V -> I2C bus signals (S_SDA) are pulled low to approx. 2V. If I increase load on S_B3V3, IC recovers to isolating mode…
  • Is this behavior expected? Is there some way how to avoid it? In datasheet it is specified V+=0V, so I guess someone was facing this problem
  • Is there a better solution for this purpose – I2C protection in case of power off?

Regards Bernd

  • 0
    TI__Expert 3880 points

    Hi Bernd,

    As you turn off the S_B3V3 rail, the decoupling capacitor will still be charged. Depending on your system it might take some time until it is discharged, through parasitic resistance to GND, below 0.4V to 0.7V. Around this voltage the powered-off protection of the TS5A63157 will work as mentioned in the datasheet.

    To speed up the discharge you could add a parallel resistor next to the capacitor. You'll have to find a balance between discharge time and power consumption.

    Can you clarify why there is S_SDA_INT on both sides of the isolation? In my understanding there would always be some voltage on S_SDA_INT, depending on S_SDA and the resistor value R2354.

    I see, that there is a second channel in you schematic, do you want to protect each channel individually or would it also be possible to combine these into a single multi-channel switch like the TMUX1511?

    Best regards,

    Ambroise

  • 0 in reply to Ambroise Suter
    TI__Genius 12760 points

    Hi Ambroise,

    thank you for the detailed feedback.

    Meanwhile I found a smarter solution with PCA9306 providing:
        • High-impedance SCL1, SDA1, SCL2, and SDA2 pins for EN = Low
        • Lockup-free Operation for Isolation when EN = Low

    Regards,
    Bernd